Diagnoses have been provided by acquiring medical images using medical imaging apparatuses such as X-ray CT devices or MRI devices and observing the medical images. As an example, diagnosis of emphysema using medical images has been carried out. Regarding the diagnosis of emphysema using medical images, it is required to specify (extract) a lung field region from a medical image. The region growing method is one method for specifying (extracting) a lung field region from a medical image. The region growing method is a method by which one pixel in a region subjected to extraction is determined and taken as the start point (seed point), and pixels considered to be within the region are consecutively extracted. As described, in the region growing method, one pixel that is present in a region subjected to extraction (for example, a lung field region) simply has to be specified. Conventionally, with reference to medical images, an observer used to visually determine one pixel that is present within a lung field region. For example, an observer used to visually verify a coronal image or an axial image in which a chest portion was represented and determine one pixel that is within a lung field region.